1. Field of the Invention
This invention relates generally to welding apparatus and methods, and more particularly to apparatus and methods for forming weld connections, and for initiating self-propagating exothermic reactions, such as in the process of forming the weld connections.
2. Description of the Related Art
Exothermic welding has become recognized as a preferred way to form top quality high ampacity, low resistance electrical connections.
Exothermic welded connections are immune to thermal conditions which can cause mechanical and compression joints to become loose or corrode. They are recognized for their durability and longevity. The process fuses together the parts or conductors to provide a molecular bond, with a current carrying capacity equal to that of the conductor. Such connections are widely used in grounding systems enabling the system to operate as a continuous conductor with lower resistivity.
Examples of self propagating exothermic reactions for exothermic welding are found in the CADWELD process and the THERMIT process. CADWELD is a trademark of ERICO International Corporation, Solon, Ohio, U.S.A., and Thermit is a trademark of Th. Goldschmidt A G, Essex, Germany. Exothermic welding mixtures are basically a combination of a reductant metal and usually a transition metal oxide. An example is aluminum and copper oxide, which upon ignition supply enough heat to propagate and sustain a reaction within the mixture. It is usually the molten metal product or the heat of this reaction, which is then used to produce a desired result. The CADWELD process produces, for example, a mixture of molten copper and aluminum oxide or slag. The higher density of the molten copper causes separation from the slag, with the molten copper usually directed by a mold to join or weld copper to copper, copper to steel, or steel to steel. The aluminum oxide slag is removed from the weld connection and discarded. Another common mixture is iron oxide and aluminum. Where only the heat of the reaction is used, the heat may be used to fuse brazing material, for example.
The exothermic reaction produces a large amount of heat. The most common way to contain the reaction, and to produce the weld or joint, has been to contain the reaction in a split graphite mold. A prior art welding apparatus 10 utilizing such a split graphite mold 12 is shown in FIG. 1. Referring to FIG. 1, the mold 12 includes an upper mold body section 14, a lower mold body section 16, and a mold cover 20. The conductors or items to be joined, such as the bars 22 and 24, are thoroughly cleaned and then placed in the appropriate location to project into a weld chamber 26 defined by the body sections 14 and 16 of the mold 12. The upper mold body section 14 includes a crucible chamber 30 above the weld chamber 26, connected to the weld chamber 26 by a tap hole 32. The mold body sections 14 and 16 are then securely closed and locked usually with a toggle clamp, and a metal disk 34 is positioned in the crucible chamber 30 over a tap hole 32. An appropriate amount of powdered exothermic material 36 is emptied into the crucible chamber 30 on top of the disk 34, and a traditional starting powder or material 40 is sprinkled over the top of the exothermic welding material 36. The starting powder 40 is essentially a much finer exothermic material. The mold cover 20 is then closed and the reaction initiated by igniting the starting powder 40 by the use of a flint igniter.
The starting powder or material 40 sprinkled on top of the exothermic material 36 has a lower ignition temperature and is easily ignited by the flint gun while the flint gun cannot normally ignite the exothermic material 36 directly. The exothermic reaction of starting powder 40 subsequently ignites the exothermic material 36. When the exothermic material 36 is ignited, the molten metal phase separates from the slag and melts through the metal disk 34. The molten metal then is directed via the tap hole 32 to the weld chamber 26 and the conductors 22 and 24 to be joined. Once the metal has solidified, the mold body sections 14 and 16 are opened and the slag is separated from the weld connection. The mold 12 may then be cleaned and readied for reuse for the next connection.
As suggested by the above, exothermic mixtures of this type do not react spontaneously and need a method of initiating the reaction. This initiation method involves generating enough localized energy to enable the reaction to begin. One method of initiating reaction is that described above, use of a starting powder and an ignition source such as a flint igniter. However, because of the starting powder's low ignition temperature and difficulties in handling and shipping, much effort has been made to find a reliable and low cost alternative ignition system for the exothermic material. A number of electrical systems have been devised which range from simple spark gaps to bridge wires or foils, to much more esoteric devices such as rocket igniters. Such efforts are seen, for example, in prior U.S. Pat. Nos. 4,881,677, 4,879,452, 4,885,452, 4,889,324 and 5,145,106. For a variety of reasons, but primarily because of power requirements, dependability, and cost, such devices have not succeeded in replacing the standard starting powder/flint gun form of initiating the self-propagating exothermic reactions. Another electrical ignition system is the system disclosed in U.S. Pat. No. 6,553,911, owned by the assignee of this application, which is incorporated herein by reference in its entirety.
In addition, there are other difficulties inherent with the welding apparatus and method described above. Aside from the difficulties in handling and shipping the starting powder 40, there may be problems in handling and shipping the bulk exothermic material 36 itself. Properly measuring the exothermic material 36 may be both time consuming and susceptible to error. Further, the graphite molds utilized in prior art apparatuses, such as the mold 12 utilized in the welding apparatus 10 shown in FIG. 1, may be costly because of the amount of graphite involved, and because of the amount of machining need to produce the passages shown in the mold 12 of FIG. 1. Finally, a process such as that described above produces undesirable residues on surfaces of the mold 12. The residues require periodic cleaning of the surfaces of the mold 12, a labor-intensive process. Even with periodic cleaning, the formation of the residues may reduce the operational life of the mold 12. Finally, the cleaning itself may cause damage to the mold, also leading to reduced operation life of the mold 12.
One alternative prior art device for containing weld material is a sealed crucible assembly 50, shown in FIG. 2. The crucible assembly 50 includes a container 52 having sidewalls 54 and a fusible bottom 56. A refractory material 60 lines the sidewalls 54 of the container 52. A pre-measured amount of weld material 64 is in the container 52. An igniter 66 has a first end 68 situated above or partially within the weld material 64, and a second end 70 protruding from the container 52. The second end 70 may be coupled to a voltage supply to produce a voltage sufficient to cause a plasma or spark at the first end 68. The plasma or spark from the igniter 66 ignites the weld material 64, causing the same type of exothermic chemical reaction as described above. A cover 74 may be placed over the top of the container 52, to maintain weld material 64 within, and/or to prevent ingress of dirt, moisture, or other undesirable substances. The crucible assembly 50 may be used in place of the metal disk 34 (FIG. 1), the exothermic material 36 (FIG. 1), and the starting powder 40 (FIG. 1), in a graphite mold similar to the mold 12 (FIG. 1), to make a weld connection. A device such as shown in FIG. 2 is sold by ERICO International Corporation under the trademark CADWELD PLUS. Further details of such a device are given in U.S. Pat. No. 6,835,910, the drawings and detailed description of which are herein incorporated by reference.
Accordingly, it will be appreciated that improved welding apparatus and methods would be desirable.